This invention relates to high power RF amplifiers, such as those employed in semiconductor plasma processing applications. The invention is more specifically related to a high power amplifier that employs high voltage large-die kilowatt power transistors. The invention can also be applied to other RF applications, such as radio communications and induction heating.
In a typical RF plasma generator arrangement, a high power RF source produces an RF wave at a preset frequency, i.e., 13.56 MHz, and this is furnished along a power conduit to a plasma chamber. The RF power is also typically provided at a fixed, known impedance, e.g., 50 ohms. An RF drive signal is generated and fed to a power amplifier, which provides the RF wave at a desired power level, e.g., 1.25 KW, 2.5 KW, 5 KW, 10 KW, etc. Depending on the application, the wave can be provided at some other frequency, such as 2.0 MHz, 4.0 MHz, 27.12 MHz, or 40.68 MHz.
The conventional plasma generator RF power amplifiers have employed RF bipolar or RF MOSFET transistors, with operating voltages (i.e., V.sub.CE or V.sub.DS) between 40 and 50 volts. These semiconductor devices have breakdown voltages (V.sub.CEO or V.sub.DSS) between 100 and 150 volts. Bipolar transistors for these RF amplifiers are commercially available as Motorola MRF448, CSF Thomson TH430, GHz Technology S200, or Philips BLW 96, at a typical cost of about $50 apiece. A 2.5 KW amplifier typically employs eight push-pull pairs in corporate parallel, with one transistor for each of the forward phase and reverse phase sections of each push-pull pair (sixteen transistors total). The RF generators based on the low voltage RF bipolar or RF MOSFET transistors have achieved very good reliability and performance for plasma applications. A typical amplifier of this construction, operating at about 45 volts rail, is rather insensitive to the particular system set up, making it very user-friendly without significant equipment interface issues, such as the ability to ignite and sustain the plasma for a desired application.
Recently, there has been an interest among RF power amplifier designers to investigate the use of high voltage MOSFET transistors in place of the low voltage RF bipolar or RF MOSFET transistors. This has been described, for example, in U.S. Pat. No. 5,726,603, granted Mar. 10, 1998. The high voltage MOSFET transistors, originally intended for use in switch-mode power supplies, have high drain-source breakdown voltages V.sub.DSS, low junction-to-case thermal resistance R.sub..theta.JC, and a low drain-to-source on resistance R.sub.DS-ON. These transistors can be formed as a rather large die, e.g., 100,000 square mils, with a high gain-bandwidth product F.sub.T. These large dies are designed as large single chip transistors with a minimum of source and gate bond wires, unlike low voltage RF bipolar and MOSFET dies, which are designed as multiple small cells with large numbers of emitter (source) and base (gate) bond wires. The high voltage, large-die design makes the high voltage MOSFET transistor better suited for high volume production, which is required for the switch-mode market. This results in a lower packaged transistor cost, typically about $17 or less for a 1KV breakdown voltage (V.sub.DSS) device in a standard TO-247 package. Amplifiers based on these devices can achieve a power gain of about 15 to 16 dB at 13.56 MHz at 350 watts power output for 160 volts DC operation. The actual stable gain into various VSWR loads depends on the circuit topology used, and can be in the range of 12 to 13 dB, depending upon the device manufacturer. The semiconductor plasma processing equipment industry is now demanding lower cost and much smaller size plasma generators, due to the small available footprint in their fabs, where fabrication space is now at a premium. High voltage MOSFETs with innovative circuit topologies, using surface mount technology and improved cooling methods, have been considered as presenting a possible solution to meet this challenge.
Any RF power amplifier is required to maintain or exceed the operational reliability of the existing equipment, i.e., the low voltage RF bipolar transistor based amplifier designs. Aside from the use of high voltage switching MOSFET transistors in switch-mode circuit topologies, the main possibility includes the use of high voltage switching MOSFET transistors with RF circuit topologies.
Within the past few years, improved high voltage MOSFET transistors have been developed for switch-mode power supply applications. The performance of these devices has improved steadily with respect to drain-source breakdown voltage V.sub.DSS, drain to source on-state resistance R.sub.DS-ON, junction-to-case thermal resistance R.sub..theta.JC, total gate charge Q.sub.q, and drain to source voltage rate of change dV/dt. The state of the art to date is represented by the background and disclosure in U.S. Pat. No. 5,726,603.